Abstract Hyperammonemia (HA) is the primary contributor of pathophysiology in the urea cycle disorders, which are a group of rare genetic diseases that affect approximately 1 in 35,000 people. HA can result in brain injury leading to irreversible intellectual and developmental disabilities, and even death. Current therapies for HA are targeted at reducing blood ammonia levels; although they can prevent death from brain edema, they are inefficient at reducing or preventing brain injury from HA. To discover and develop treatments that prevent brain injury from HA and/or promote recovery, we performed a phenotypic screen of >10,000 chemicals for the protection of zebrafish from ammonia toxicity and identified 16 compounds that prolonged the survival of fish exposed to high ammonia concentrations and allowed the fish to maintain normal, or close to normal locomotor response to light-dark cycle. Evaluation of these candidate drugs in both preclinical and clinical settings requires validated biomarkers for monitoring brain injury from HA. Absence of validated systemic biomarkers that can be used to monitor brain injury during acute HA is a major barrier in the search for therapies aimed at protecting brain from ammonia toxicity. This challenge will be addressed by the following proof-of-concept specific aims: 1. To determine whether plasma concentrations of brain-specific proteins correlate with brain injury from HA. These studies will be carried out with our animal model of inducible HA, the homozygous N- acetylglutamate synthase knockout (NAGSko) mouse that survives into adulthood and reproduces when supplemented with N-carbamylglutamate and citrulline, and develops HA when supplementation is withdrawn. HA will be induced in the NAGSko mice followed by measurements of plasma ammonia, S100B, neuron- specific enolase (NSE), and ubiquitin C-terminal hydrolase L1 (UCHL1) at baseline, during acute HA and one week after recovery from the HA episode. 2. To determine whether concentrations of S100B, NSE and UCHL1 in the blood correlate with quantitative MRI measurements of brain injury due to HA. We will measure ammonia S100B, NSE and UCHL1 in the blood of HA NAGSko mice, and employ magnetic resonance modalities to calculate fractional anisotropy before and during acute HA episode. 3. To characterize the chronology of S100B, NSE and UCHL1 blood levels in response to an HA brain insult in affected UCD patients. We will conduct a pilot study in patients with proximal UCD (either ornithine transcarbamylase or carbamylphosphate synthase 1 deficiencies) to determine whether plasma S100B, NSE and UCHL1 correlate with blood ammonia during an acute HA episode. The proposed studies will deliver the validated biomarkers required for monitoring brain injury from acute HA. These biomarkers will be essential for measuring the effects of novel interventions aimed at preventing brain injury from high ammonia concentrations.